Mary Warschauer was 21 when the second world war broke out, and had been working in the Air Ministry in London for a year. Originally she was from Newcastle, a bright girl who, she tells me, had gained her place by competitive examination. At 90, blind and living in a bungalow in Dorset, she still sparkles, makes me tea and sandwiches (with the crusts cut off) and hands me a sheaf of typed notes of her wartime experiences, because she says these days she loses track as she speaks. Her blindness is evidently no bar to typing.

For the first four years of the war, Mary worked as a teleprinter operator in the basement of an office block in King Charles Street, just off Whitehall, which also housed the cabinet war room. She was in the cipher room, next door to where Churchill and his cabinet met. Once, she tells me, she was on the staircase with a large joke cigar in her mouth that lit up when you puffed on it, imitating the great man, when he appeared. "I'm sure he was grinning," she says, "and he always used to nod as we passed." Initially, she bunked down in the block, but after a few months moved to a theatrical boarding house in Gower Street, a mile north of Whitehall. During the blitz, she often had no option but to walk to work, shrapnel pinging around her.

"I had to go to work both days and nights," she recalls, "and when I went at night I was going in the blackout. Sometimes a bus would come and sometimes it wouldn't. It was quite a walk in the dark. Once I remember tripping over a sandbag and squashing the pies I'd bought in the mud. We didn't get any pies that night." Did she realise she was at the heart of the war effort? "Definitely," she says. "I knew all about things that were happening before they happened, and I couldn't say a word."

Like many of the blitz survivors I meet, she disliked taking refuge in tube stations. "There were shelters, and my boyfriend and I went into some occasionally. But I was horrified by the underground because there were people living on the platforms. They used to come down there regularly every night, and they had camp beds, with babies next to dirty old men – all sorts together."

Thoughts of her boyfriend, whom she didn't marry but did keep in touch with until his death last year, conjures up a memory of sitting with him in Hyde Park during an air raid, as anti-aircraft fire clattered down on the ground beside them. "One piece fell between the two of us," she says, "red-hot." If it had struck you it could have killed you, I say. "Yes, but we were young. We didn't think of that."

I hear this sentiment repeatedly: the young felt they were indestructible, feeding off the excitement of the moment. "Once the all-clear sounded," Mary says, "we walked up to Marble Arch, and that's something I'll never forget. There wasn't a sign of traffic, so the two of us did the waltz down Oxford Street, then went hand-in-hand singing our heads off down Bond Street as far as Piccadilly. We enjoyed life." And defied death. "If I'd had children it would have been a different case, But everybody got on with the day, cleared up as quickly as they could, and tried to carry on as if they didn't care. And I didn't care."

She recalls coming out of the ministry when the all-clear sounded after one ferocious attack, to be confronted with an extraordinary sight. "We went to go over Westminster Bridge, but all the ambulances were still there, and people were yelling and shouting. You could see right up the river, and all you could see were these skeletons of buildings, and everything in flames. But what remained in my memory for ever was the river, which looked like fire. All the burning buildings were reflected in the water, like a river of flame. It was beautiful and awesome, yet tragic."

Her memories are a mixture of the poetic and the prosaic. The funniest concerns her search for a loo in central London after she had taken some castor oil and was in urgent need. She tried a succession of public toilets, but they had all been bombed out. Eventually she got to Hyde Park Corner, where a recent raid had destroyed a row of houses. "Up in the trees," she says, "was a loo that had been blown out of one of the houses, and I couldn't use it." Eventually she found relief in Knightsbridge. "There are so many things I can remember that were funny. We had more humour then."

She recalls one incident that did pierce that sense of indestructibility. "The only time I ever remember being frightened – and it was only for a few seconds – was when I was going home from duty along Victoria Street one evening, and a burly French navy man crossed over the road, pushed me into a doorway and tried to get off with me. I was saying 'No, no, no', and I was that furious I gave him an almighty slap across the face. I suddenly realised what I'd done, and thought, What's going to happen to me now? I had plaits round my head in those days, and he took me by the shoulders and shook me and shook me till my hair fell down my back. He was hissing like a cat – I'd never heard a human being do that. But after shaking me he went off, and I went on to Victoria with my hair still down my back."

Did she ever doubt Britain would prevail? "No," she says. "Churchill used to come along that corridor. He was nearly always in a blue siren suit, and something about his presence made you feel comfortable and secure. He had an aura. We all felt the same. He used to go up on the Air Ministry roof [during raids] when he shouldn't have done."

The blitz lasted for nine months in London. How did she cope? "We just got used to it. You took it in your stride. But I realised afterwards it must have had some effect, because after the war I was walking along one day when a train went overhead, and I suddenly found myself down on my hunkers. I thought we were under attack again."

Mary also says the war left her with a lifelong need to answer the phone at any time of day or night – in case it was a crucial message. After the war, she worked as a civil servant in the Ministry of National Insurance, but the habits of the Air Ministry, the intensity of war, never left her.

Footage shot by Alfred Coucher, wartime mayor and chief air raid warden of Marylebone, west London. It has been released by Westminster city council after lying unseen in the Coucher family's attic for 70 years